A longitudinal cohort study on the effects of reducing farm-level antimicrobial use on Escherichia coli antimicrobial resistance on dairy farms in Québec, Canada.

The effects of interventions that restrict antimicrobial use (AMU) in food production animals on antimicrobial resistance (AMR) remain unclear. We have already established a portrait of AMU and AMR on dairy farms in Québec before a regulation, implemented in 2019, that restricts the use of category I antimicrobials in food production animals. In the current study, involving 84 dairy farms in Québec, we assessed the effects of farm-level changes in AMU on farm-level AMR. Using an observational cohort study design, we gathered AMU data through veterinary invoicing software 2 years before and 2 years after the regulation was implemented. We sampled calves' and cows' feces and the manure pit on each farm during the 4 years. We tested the indicator bacterium Escherichia coli for susceptibility to 20 antimicrobials and evaluated putative extended spectrum β-lactamase/AmpC E. coli growth. Overall use of antimicrobials (all antimicrobials and all administration routes combined) significantly decreased after regulation implementation. Specifically, the 2 types of use that were implicated in the decrease were injectable third-generation cephalosporins and intramammary polymyxin B. A farm's probability of experiencing AMR appeared to increase with higher AMU during the same period, specifically when the use of all antimicrobials combined was considered. A farm's pre- versus post-regulation AMU variation (i.e., reduction, increase, stable) had no apparent effect on its post-regulation AMR. In conclusion, regulation and possibly other concomitant factors had a positive influence on the judicious use of critical antimicrobials and AMR. Moreover, a farm's AMR profile was better explained by its current than by its historical AMU management.